1. Field of the Invention
The present invention relates to an intake- and/or exhaust-valve timing control system which is optimally, adapted for use in internal combustion engines. More particularly, the invention relates to a system which is variably capable of controlling intake- and/or exhaust-valve timing depending on the operating state of the engine, for example, the magnitude of engine load and/or engine speed.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various intake- and/or exhaust-valve timing control systems for internal combustion engines for generating optimal engine performance according to the operating state of a vehicular engine.
As is generally known, valve timing is usually determined such that optimal engine performance is obtained; however, a predetermined valve timing is not suitable under all operating conditions. For example, when an engine is operating within a range of low revolutions, higher torque will be obtained with an intake-valve timing earlier than with a fixed, predetermined valve timing.
Such a conventional intake- and/or exhaust-valve timing control system for internal combustion engines has been disclosed in U.S. Pat. No. 4,231,330. In this conventional valve timing control system, a cam sprocket having a driven connection with an engine crankshaft is rotatably supported through a ring gear mechanism at a front end of the camshaft. The ring gear mechanism includes a ring gear having an inner toothed portion engaging another toothed portion formed on the front end of the camshaft and an outer toothed portion engaging an inner toothed portion formed on the inner peripheral wall of the cam sprocket. In this manner, the ring gear rotatably engages between the cam sprocket and the camshaft. The ring gear is normally biased in the axial direction of the camshaft by spring means, such as a coil spring. At least one of the two meshing pairs of gears is helical. The result is that axial sliding movement of the ring gear relative to the camshaft causes the camshaft to rotate about the cam sprocket and therefore the phase angle between the camshaft and cam sprocket (and consequently, the phase angle between the camshaft and the engine crankshaft) is varied relatively. The ring gear moves as soon as one of the two opposing forces acting on it, namely the preloading pressure of the above spring means or the oil pressure applied from the oil pump through the flow control valve to the ring gear, exceeds the other. The conventional valve timing control system also includes an end disc locked on the front end of the camshaft by threading such that the end disc hermetically closes the front opening of the substantially cylindrical cam sprocket in an air-tight fashion. As is well known, when a crankshaft is connected through a timing chain or a timing belt to a camshaft, the phase angle between the crankshaft and the camshaft must be set to a predetermined value to obtain desirable valve timing. For this reason, timing marks may be indicated on the crank sprocket, the timing chain, and/or the cam sprocket for instance. However, in the conventional valve timing system as described, when the end disc is screwed into the inner threaded portion formed in the center of the front end of the camshaft, the relative phase angle relationship between the cam sprocket and the camshaft is varied and as a result, the phase angle between the crankshaft and the camshaft is offset from the predetermined phase angle as well. Therefore, the phase angle between the camshaft and the cam sprocket must be adjusted after threading the end disc into the front end of the camshaft. Furthermore, in conventional valve timing control systems, the relative phase angle between the inner and outer toothed portions of the ring gear is not always set to a particular value. That is, the inner toothed portion of the ring gear is independently formed irrespective of the phase angle of the outer toothed portion. Consequently, in conventional valve timing control systems, phase angle adjustments are troublesome and time consuming.